Rare earth sintered magnets, such as R-T-B-based sintered magnets (R means at least one of rare earth elements (concept including yttrium (Y)), T means iron (Fe) or a combination of iron and cobalt (Co), and B means boron) and samarium-cobalt-based sintered magnets are widely used because of excellent magnetic characteristics such as a residual magnetic flux density Br (hereinafter sometimes simply referred to as “Br”) and a coercive force Hcj (hereinafter sometimes simply referred to as “Hcj”).
Particularly, R-T-B-based sintered magnets are used for various applications, including various motors such as voice coil motors of hard disk drives, motors for hybrid vehicles, motors for electric vehicles, and home electric appliances, because of the highest magnetic energy product among various conventionally known magnets and the affordable low price. There have recently been demands for more improvement in magnetic characteristics of rare earth sintered magnets such as R-T-B-based sintered magnets for the sake of size reduction and weight reduction or increase in efficiency for various usages
The R-T-B-based sintered magnet includes, as a main phase, an R2T14B phase which is a ferromagnetic phase, and also has a structure in which a non-magnetic low-melting point R-rich phase of a concentrated rare earth element (R) coexists. There have been known, as the method for improving magnetic characteristics of the R-T-B-based sintered magnet, methods in which (1) an R2T14B phase is refined, (2) the orientation degree of an R2T14B phase is enhanced, (3) the amount of oxygen is reduced, and (4) a ratio of an R2T14B phase is increased.
In the production of numerous rare earth sintered magnets including an R-T-B-based sintered magnet, it is possible to use an ingot obtained by melting (fusing) raw materials such as metals and casting the molten metal into a mold, or an alloy powder having a predetermined particle diameter obtained by grinding a raw material alloy cast material with the desired composition such as a strip obtained by a strip casting method. The alloy powder is subjected to press molding (press molding in a magnetic field) to obtain a molded body (green compact) and also the molded body is sintered to produce numerous rare earth sintered magnets including an R-T-B-based sintered magnet.
In the case of obtaining an alloy powder from a casting material, in many cases, steps to be used are two grinding steps of a coarsely grinding step of grinding into a coarse powder having a large particle diameter (coarsely ground powder) and a finely grinding step of further grinding the coarse powder into an alloy powder having a desired particle diameter.
The method of press molding (press molding in a magnetic field) is roughly classified into two methods. One is a dry molding method in which the obtained alloy powder is subjected to press molding in a dry state. The other one is a wet molding method mentioned, for example, in Patent Document 1, in which an alloy powder is dispersed in a dispersion medium such as oil to prepare a slurry, and the alloy powder is supplied in a cavity of a mold in a state of the slurry, followed by press molding.
Furthermore, the dry molding method and the wet molding method can be roughly classified into two methods, respectively, according to a relation between the pressing direction at the time of pressing in a magnetic field and the direction of the magnetic field. One is a perpendicular magnetic field molding method (also referred to as a “transverse magnetic field molding method”) in which the direction of compression preformed by a press (pressing direction) is orthogonal to the direction of the magnetic field applied to an alloy powder. The other one is a parallel magnetic field molding method in which the pressing direction is in parallel with the direction of a magnetic field applied to an alloy powder (also referred to as a “longitudinal magnetic field molding method”).
The dry molding method is often employed since a molding machine has a comparatively simple structure, and steps such as removal of dispersion medium during press molding and removal of the dispersion medium from the molded body are not needed. Particularly, according to the perpendicular magnetic field molding method, since the pressing direction is orthogonal to the magnetic field application direction, press molding can be performed without drastically disturbing the orientation of the alloy powder oriented to the magnetic field application direction, thus enabling production of a molded body having high orientation degree of an R2T14B phase. Meanwhile, according to the parallel magnetic field molding method, since the pressing direction is in parallel with the magnetic field application method, the orientation of the alloy powder is likely to be disturbed at the time of press molding, and thus the R2T14B phase exhibits low orientation degree as compared with the perpendicular magnetic field molding method. Therefore, in the dry molding method, the perpendicular magnetic field molding method is mainly used, and only a product having a shape such as disc, ring or thin sheet, which is difficult to mold by the perpendicular magnetic field molding method, is produced by the parallel magnetic field molding method.
However, in the dry molding method, exposure of an alloy powder to the atmospheric air is unavoidable when the alloy powder is supplied in a cavity and press molding is performed, and also a molded body is exposed to the atmospheric air when the molded body is removed after completion of press molding, thus causing an increase in the amount of oxygen of the molded body, leading to deterioration of magnetic characteristics. It is difficult to avoid producing large friction between alloy powders or between an alloy powder and a mold, and also there is a limitation on increase in orientation degree of the R2T14B phase because of large resistance when the alloy powder is rotated and orientated by the applied magnetic field.
Meanwhile, there is a need for the wet molding method to perform supply of a slurry and removal of a dispersion medium, and thus the structure of a molding machine becomes comparatively complicated. However, oxidation of the alloy powder and the molded body is suppressed by the dispersion medium, thus enabling reduction in the amount of oxygen of the molded body. The dispersion medium exists between alloy powders at the time of press molding in the magnetic field, and thus the alloy powder can rotates more easily in the magnetic field application direction because of weak restriction due to a friction force. Therefore, higher orientation degree can be obtained. Thus, there is an advantage that it is possible to obtain a rare earth sintered magnet which is more excellent in magnetic characteristics as compared with the dry molding method.
In this way, it is possible to obtain high orientation degree and excellent oxidation suppressing effect as compared with the dry molding method when the wet molding method is used, and the R-T-B-based sintered magnet thus obtained tends to have higher magnetic characteristics. High orientation degree and excellent oxidation suppressing effect obtained using the wet molding method can be obtained in not only this R-T-B-based sintered magnet, but also other rare earth sintered magnets.
However, the wet molding method also has the following problems.
In the wet molding method, when the slurry is charged in a cavity and press molding is performed in the magnetic field, there is a need for most of a dispersion medium (oil, etc.) in the slurry to be discharge out of the cavity. Usually, at least one of an upper punch and a lower punch is provided with a dispersion medium outlet and, when the volume of the cavity decreases by the movement of the upper punch and/or the lower punch to pressurize the slurry, the dispersion medium is discharged through the dispersion medium outlet. In this case, since the dispersion medium in the slurry is filtered and discharged from the portion close to the dispersion medium outlet, a layer called a “cake layer” having increased concentration (high density) of the alloy powder is formed at the portion close to the dispersion medium outlet in an initial stage of press molding.
As the upper punch and/or the lower punch move(s) and press molding proceeds, much more dispersion medium is filtered and discharged, and thus an area of the cake layer spreads in the cavity. Finally, the cake layer (having high density of the alloy powder (low dispersion medium concentration) spreads all over the cavity, resulting in achieving bonding between the alloy powders (comparatively weak bonding) to obtain a molded body.
In the initial stage of press molding, when the cake layer is formed at the portion close to the dispersion medium outlet (upper portion and/or lower portion in the cavity), the direction of the magnetic field tends to be curved in the perpendicular magnetic field molding method.
The cake layer exhibits increased magnetic permeability as compared with the portion other than the cake layer of the slurry (portion with less amount of the alloy powder per unit volume) because of high density of the alloy powder (large amount of the alloy powder per unit volume), thus causing focusing of the magnetic field in the cake layer. This means the fact that, even if the magnetic field is applied approximately perpendicularly to the cavity side surface outside the cavity, the magnetic field is curved to the cake layer inside the cavity. Therefore, since the alloy powder is oriented along this curved magnetic field, the portion with curved orientation exists in the molded body after press molding, leading to a decrease in orientation degree in the single molded body, thus failing to obtain sufficient magnetic characteristics in the sintered magnet.
A problem of deterioration of magnetic characteristics of the rare earth sintered magnet due to curved magnetic field becomes noticable as the size of the cavity in the magnetic field application direction increases, for example, more than 10 mm.
Meanwhile, in the parallel magnetic field molding method, since the magnetic field is applied to the direction parallel to the pressing direction, i.e. the direction parallel to the direction from the upper punch toward the lower punch, even if the cake layer is formed at the portion close to the dispersion medium outlet of the upper punch and/or the lower punch, the magnetic field travels straight toward the inside of the cake layer from the portion where the cake layer does not exist without being curved. Therefore, like the perpendicular magnetic field molding method, the magnetic field is not restricted by the size of the cavity in the magnetic field application direction.
However, when the size of the cavity in the magnetic field application direction increases, since a distance between coils serving as a generation source of the magnetic field increases, the strength of the magnetic field applied into the cavity decreases, and thus the orientation degree of the alloy powder decreases. Therefore, the magnetic field strength must be increased in case the size of the magnetic field application direction is increased. In order to solve a problem that the orientation of the alloy powder is likely to disturbed at the time of press molding since the pressing direction is in parallel with the magnetic field application method, it is effective to increase the magnetic field strength.
However, even if the magnetic field strength is increased, the desired magnetic characteristics cannot be obtained sometimes. Particularly, in case an attempt is made to obtain a long and large-sized molded body having a large size of the cavity in the magnetic field application direction, variation in density in each portion of the molded body tends to occur. This is a problem peculiar to the wet molding method, and the same problem may occur in the perpendicular magnetic field molding method. When variation in density in each portion of the molded body occurs, there arise problems that cracks generate in the molded body at the time of removing the molded body after press molding, and that cracks generate due to shrinkage at the time of sintering.
Under these circumstances, a parallel magnetic field molding method using a wet molding method is known in documents such as Patent Document 1, however, the parallel magnetic field molding method was not used in the production of a long molded body or a large-sized molded body in which the size of the cavity (depth size of the cavity) in the magnetic field application direction has a large value of more than 10 mm in actual manufacturing site. In other words, there has never been produced a rare earth sintered magnet, which is obtained from a molded body in which the size of the cavity (depth size of the cavity) in the magnetic field application direction is more than 10 mm, and has uniform and high magnetic characteristics, by the wet molding method.
There has conventionally been produced a molded body having a large size in the magnetic field application direction mainly by the perpendicular magnetic field molding method using a dry molding method. For example, as disclosed in Patent Document 2, a magnet for voice coil motors of a hard disk drive has been produced by subjecting a long molded body having a shape, a cross section of which is composed of an approximately arc-shaped outer circumference, an approximately arc-shaped inner circumference, and a pair of side circumferences connecting between the outer circumference and the inner circumference (hereinafter referred to as an “approximately arc shape”), to press molding, followed by sintering and further slicing in the direction orthogonal to the magnetic field application direction.
However, as mentioned above, in the dry molding method, the amount of oxygen of the molded body increases to cause deterioration of magnetic characteristics, and also there is a limitation on an increase in orientation degree of the R2T14B phase. Also in the perpendicular magnetic field molding method using the dry molding method, there is a limitation on the size of the magnetic field application method.
Therefore, although a comparatively simple shape such as rectangular parallelepiped can be produced by the method, it is difficult to form a complicate shape having an approximately arc-shaped cross section. Even if the complicate shape can be formed by the method disclosed in Patent Document 2, sufficient magnetic characteristics could not be often obtained.
Furthermore, it was impossible to produce a long molded article, which is used as a magnet for voice coil motors of a hard disk drive in recent days, having a large size in the magnetic field application direction and also having a complicated cross-sectional shape in the direction orthogonal to the magnetic field application direction, for example, a shape whose cross section has an approximately arc shape, protrusions being formed on at least a part of an outer R surface (approximately arc-shaped outer peripheral surface), an inner R surface (approximately arc-shaped inner peripheral surface) and an arc end face, by the dry molding method.
Patent Document 1: JP 7-57914 A
Patent Document 2: JP 2001-58294 A